The art of photolithography forming a fine pattern on a substrate is often utilized in manufacturing kinds of semiconductor devices and display devices. In photolithography, photoresist is coated on a substrate, which is hereinafter called “circuit substrate”. The photoresist is exposed to light of a circuit pattern. Then the circuit pattern is transferred by carrying out development of the photoresist and etching through the developed photoresist.
The photomask as master piece has the structure where the circuit pattern is formed on a transparent substrate, hereinafter called “mask substrate” for discrimination from circuit substrates. Photomasks are fabricated from products called “mask blanks”. A mask blank is fabricated by fully covering one side of a mask substrate with a shading film.
In photolithography, the problem of the photomask charge-up has been arising. Although the shading film, which may be chrome film or chrome compound film, in a photomask is conductive, the mask substrate is made of glass, i.e., dielectric, which can be easily charged up. Especially, when a photomask is used for so-called contact exposure, it can be charged up easily because it is contacted onto a circuit substrate. The charge up can happen when the photomask is detached from the circuit substrate after the exposure, i.e., so-called “contact-and-separation charge-up”.
When too much static electricity is induced on a photomask, discharge, i.e., insulation breakdown, would take place by any cause. For example, the shading film pattern on the mask substrate may have a part of “island”, that is, a part completely isolated from other parts. Potential at this island part could be much different form the other parts as result of that the photomask is charged up. When the potential difference is enlarged, a discharge may take place along the surface of the mask substrate. Otherwise, a discharge may take place between the mask substrate and an element located nearby or during maintenance.
One of problems caused by the photomask gaining charge is deformation of the shading film pattern. For example, the shading film pattern could be chipped off by impact of a discharge. Deformation of the pattern could cause decrease of the yield rate, because it is transferred to a circuit substrate.
A discharge by an excessive charge-up tends to take place at a keen part of a shading film (conductive film) pattern, e.g., right angle corner. Rounding the corner helps to prevent the discharge and could be the solution for this. However, such transformation (rounding) of the pattern is in many cases prohibited because it is the master fine pattern to be transferred to a circuit substrate.
As a structure preventing the photomask charge-up, it is considered that an ITO (indium tin oxide) film, which is the transparent conductive film, is formed on a mask substrate, and a shading film is patterned thereon. This structure is disclosed in JP2008-241921-A1. However, ITO films have only low tolerances against etchant liquids in patterning shading films. This brings the problem that an ITO film is made thinner through etching by the etchant. Otherwise, the tolerance against the cleaner liquid in a cleaning step could be questioned. When an ITO film is made thinner by chemicals, the sheet resistance thereof increases because of the volume reduction, and then could lose the required grounding function.